(1) Field of the Invention
The present invention relates to locating and tracking the source of short duration periodic signals. More particularly it relates to locating and tracking these sources utilizing time-difference-of-arrival (TDOA) validation and association measures.
(2) Description of the Prior Art
In passive sonar, as shown in FIG. 1, it is well known to deploy an array of acoustic sensors 10 from a vessel 12 in a body of water 14. Each acoustic sensor or hydrophone 16 on array 10 is positioned a known distance from another hydrophone 16 along the array line or other such structure. Hydrophones 16 are capable of receiving acoustic signals 18 from sources such as 20. Array 10 is typically joined to a signal processor 22 on board vessel 12. Once such a signal has been received at several hydrophones 16, signal processor 22 utilizes the time the signal 18 arrives at each hydrophone 16 to calculate a time-difference-of-arrival (TDOA) between several pairs of hydrophones. The signal processor 22 has a sound velocity profile from either calculation, a database or a determination giving the speed of sound in the body of water 14, and separation between the hydrophones 16 is also known. Using the known distances, the sound velocity profile and the TDOAs from the array, the signal processor can utilize hyperbolic calculations to determine the location of source 20.
Source 20 can be a variety of sources, such as vessels, pingers, marine mammals or the like. Marine mammals and other biological sources often make short duration periodic signals such as clicks. It is common to locate whales by measuring the time difference for the same click arriving at two spaced hydrophones.
There are several deficiencies with the current state of the art. First, many false TDOA measurements are created with TDOA estimates that are found using correlation. When data from two hydrophones are correlated, the noise that is independently received at each sensor does not correlate well and produces very little or no output. However, when a common signal is present at the two hydrophones, it will cause a correlation peak at the TDOA estimate. When the signal is weak, it is difficult to separate the valid TDOA measurements from fluctuations caused by noise or false peaks spaced from the true peak by the period of the repeating click sequence.
Another issue with the current state of the art arises when more than one source is present with each source having a similar signal. This can occur when the sources are whales in a pod. In this case, a set of hyperbolic solutions will be created for each source and many false intersections are created when the hyperbolic solutions of one source intersects the solutions of another source. It is impossible to know that these intersections are false unless additional information is made available. One way to resolve these ambiguities is to only accept intersections of hyperbolic solutions from hydrophone pairs that share one hydrophone (referred to as hydrophone triples). This results in a fixed relationship between the TDOA measurements that can be checked for consistency.
The problem with the hydrophone triple method is that it does not lend itself well to soft decisions, in other words, whether the TDOAs are related or not related. Granted, it could be adapted to a soft measure simply by measuring how well the fixed relationship holds. However, to get a meaningful measure of association, it is necessary to compare the full set of click arrivals that went into making the TDOA estimate.
As can be seen, there is a need for improved methods for detection, TDOA determination and TDOA association for localization of sources such as marine mammals and other short duration periodic signal sources.